Reactive Plots with Shiny: A Deep Dive into User Input and Data Accumulation

In this article, we will explore how to create reactive plots in Shiny using user input. We will dive into the world of event-driven programming and learn how to update our plot in real-time as the user interacts with it.

Understanding the Basics of Shiny

Before we begin, let’s cover some basic concepts that you may not be familiar with:

• Shiny: A R package for building web applications using R. It allows users to create interactive web pages and plots.
• UI/Server Architecture: In Shiny, the UI (User Interface) is where the user interacts with the application, while the Server is responsible for processing the input and generating the output.
• Input/Output Objects: These are the core components of a Shiny app. Input objects represent user interactions, such as sliders or text inputs, while output objects represent the results of the server’s calculations.

The Challenge: Making the Plot Reactive to User Input

Our goal is to make the plot reactive to the user slider input periods. We want the plot to update in real-time whenever the user changes the value of periods.

Here’s an example code snippet that demonstrates how to achieve this:

library(shiny)

### Define vectors ##############################################################
beginBal       <- 1000
yield_vector   <- c(0.30,0.30,0.30,0.30,0.30,0.28,0.26,0.20,0.18,0.20)
npr_vector     <- c(0.30,0.30,0.30,0.30,0.30,0.30,0.30,0.30,0.30,0.30)
mpr_vector   <- c(0.20,0.20,0.20,0.20,0.20,0.20,0.20,0.20,0.20,0.20)
default_vector <- c(0.10,0.10,0.10,0.10,0.10,0.09,0.08,0.07,0.06,0.05)

### UI ####################################################################
ui <- fluidPage(
  sliderInput(inputId = "periods",
              label = "Periods:",
              min = 1,
              max = 10,
              value = 5),
  plotOutput(outputId = "balancePlot")
) # close fluid page

### Server #############################################################
server <- function(input, output){

  ### Generates data for plotting ###############################################
  f      <- function(x,y){x*(1+npr_vector[y]-mpr_vector[y]-default_vector[y]/12)}
  res    <- Reduce(f,seq(input$periods),init=beginBal,accumulate=TRUE)
  b      <- head(res,-1)
  endBal <- res[-1]
  
  ### Output the plot #############################################################
  output$balancePlot <- renderPlot({
    plot(endBal)
  })
}

shinyApp(ui = ui, server = server)

In this code snippet, we define a sliderInput object called periods, which represents the user slider. We also define a plotOutput object called balancePlot, which represents the plot that will be generated by the server.

In the server function, we use the Reduce function to accumulate the values of x and y over the range of seq(input$periods). This is where the magic happens!

When the user changes the value of periods, the input$periods object is updated, which triggers a new calculation in the server. The resulting values are stored in the res variable, and we use these values to generate the plot.

Conclusion

In this article, we have learned how to create reactive plots in Shiny using user input. We covered some basic concepts of Shiny, including the UI/Server architecture and input/output objects. We also explored how to make a plot reactive to user input by using the Reduce function to accumulate values over a range.

I hope this article has provided you with a deep understanding of how to create interactive web pages and plots in Shiny. Happy coding!

Last modified on 2025-03-11